In process engineering the general practice for the preparation of products involves working with ratios of quantities as set forth in a recipe. Some raw materials are added in gram-range quantities, others in large quantities. In addition, some raw materials require high shear rates when being added, others low shear rates. When producing dispersions, in particular when producing emulsions, in order to achieve a final product with the desired properties in terms of the size distribution of the disperse phase, flow properties and stability of the product relative to thermal and mechanical stresses as well as changes over time, it is extremely important that the necessary steps for adding the inner phase in the outer phase are defined and reliably implemented in terms of the process technology both during the dispersion as well as the stabilization of the obtained product. Dispersions, in particular emulsions, are produced industrially by various processes. The selected process depends on the type of the dispersion as well as on the fineness of the dispersed or particulate phase that is suitable for achieving a stable dispersion over the required time period. A stable dispersion is defined as a material system having a particle size distribution of the dispersed phase and/or flow properties, in particular, the viscosity of which does not change substantially over the prescribed time period.
Partial over-concentration is often encountered especially in emulsions and suspensions with a high disperse-phase portion; i.e., localized and/or temporal deviations regarding particle density in the surrounding fluid. According to conventional methods, such over-concentrations can only be homogenized by operation with extended mixing and dispersion times. The risk for the occurrence of over-concentrations is intensified for containers with low filling levels because the commonly used devices set the liquid that is inside the container into a rotating motion. This causes a reduction of the actual energy quota that is introduced into the fluid. Said effect is especially pronounced under vacuum conditions when the drawing-in of the fluid from the container is disrupted.
Consequently, it is necessary to maintain a certain minimum filling level inside the container in order to be able to feed the premix with the desired defined properties to the rotor-stator dispersion machine and to prevent, in particular, any aspiration of air into the rotor-stator system. To achieve full dispersion power this minimum batch volume is approximately 35% of the maximum filling level of the container for the current rotor-stator dispersion machines that are commercially available. Only if these conditions are met, will there no longer be any detectable differences in the droplet-size distributions between partial and full batches.
The usual rotor-stator dispersion machines therefore come with the disadvantage of having a relatively high minimum filling level. This is why the possibilities for varying batch quantities that are to be processed by a machine, so-called batch size ranges, are very limited. In addition, the minimum filling level must also be maintained when running the cleaning applications of the machines; thus, large quantities of cleaning agent are needed. This means a relatively large quantity of fluid is always being moved around inside the respective installation and the total power that is introduced in the fluid is dissipated inside this large volume, whereby the achieved energy density inside the product is relatively small. This can result in inefficient conditions for the dispersing action. Moreover, there is a risk that the particles and/or the drops of an emulsion coalesce.